A personal wireless communication device (herein called a “wireless device”) such as a cellular telephone includes various components, such as modulators, demodulators, frequency synthesizers, etc., that synchronize their operations to a reference clock signal. The reference clock signal may be generated from a sinusoidal reference signal that is provided by an oscillator of the wireless device.
The frequency of the sinusoidal reference signal (and thus, the frequency of the reference clock signal) is not fixed, but rather, a baseband controller of the wireless device may continually “fine tune” the frequency to synchronize communication between the wireless device and a base station. Thus, the wireless device typically controls the frequency of the oscillator in a feedback loop in response to the frequency of an incoming wireless signal from the base station.
FIG. 1 depicts a conventional system 10 that may be used in a wireless device to control the oscillation frequency of a crystal oscillator 24. The oscillator 24 provides a sinusoidal reference signal (at its output terminal 26) that is converted by other circuitry (not shown) into a reference clock signal for the wireless device. The system 10 includes a baseband control circuit 12 that receives an indication of the frequency of the incoming wireless signal and generates an analog signal (called “AFC”) in an attempt to match the oscillation frequency of the oscillator 24 to the incoming frequency. The frequency of the incoming wireless signal may be determined, for example, by examining a particular time segment of the incoming wireless signal.
Based on the determined frequency of the incoming wireless signal, the baseband control circuit 12 typically generates digital data that indicates a frequency for the oscillator 24 (and thus, the frequency for the reference clock signal). A digital-to-analog converter (DAC) 14 of the wireless device produces the AFC analog signal in response to the digital data. The AFC signal propagates through a low pass filter 18 before reaching a varactor 22, a component that is coupled to the oscillator 24 to control the oscillator's frequency (and thus control the frequency of the reference clock signal).
The varactor 22 has a capacitance that is controlled by its DC reverse-bias operating point. Therefore, the AFC analog signal controls the DC reverse-bias and capacitance of the varactor 22. Because the varactor 22 is coupled to a resonant tank (not shown in FIG. 1) of the oscillator 24, the capacitance of the varactor 22 controls the oscillation frequency of the oscillator 24 and thus, controls the frequency of the reference clock signal. Analog control, however, may be undesirable in certain implementations due to size, power, and noise or interference considerations. Accordingly, other ways of controlling an oscillator's frequency are needed.